Update Store getConfig() to respect valid false return value

[Jeroenll]

Description

Using a config setting Yes/No will return string '0' when No is saved. Method will therefore fetch default config value because '0' equals false. Removed the ScopeConfigInterface::SCOPE_TYPE_DEFAULT because it's the default value.

Fixed Issues (if relevant)

1. None I could find.

Manual testing scenarios

1. Add config value to Store module in both default and store view scope.
2. Set config on store scope to No.
3. Value '0' is returned and value from default scope is fetched.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)